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Objective Reviews & Commentary - An Engineer's Perspective

May 9, 2011

AMB Mini3 DIY Headphone Amp

DIY? There’s a significant group of audiophiles into Do-It-Yourself (DIY) projects. And DIY seems to be on an upswing and gaining popularity. Headphone amps, being relatively simple, are especially DIY-friendly. There’s a strong emotional element to DIY that can put a huge smile on your face. These designs are not subject to commercial constraints allowing higher-end components to be used. Some believe this gives DIY a big advantage. But, as I’ve said before, a good chef can do more with everyday ingredients than an amateur can with expensive exotic foods. Implementation is everything. So how does the Mini3 measure up? Is it a good design worthy of a DIY’ers time and money?

THE MINI3: As I see it, the Mini3 is an attempt to improve on the basic Cmoy while still minimizing the cost, size and complexity. It comes in “high performance” and “extended run time” editions which only differ in the op amps used. I tested the high performance version which is touted as driving low impedance loads with an impressive 300 mW into 33 ohms. You can buy a Mini3 fully assembled for around $180 which is rather expensive for essentially a Cmoy amp or you can buy the circuit board, parts, case, front/back panels, etc. for around $100. AMB makes several strong claims such as:

“High-end sonics from a tiny package -- the Mini³ is carefully designed for excellent performance, rivaling amplifiers many times its size and cost.”

AMB LABS (revised 5/14): There used to be a paragraph here saying many favorable things about AMB Labs “hi-fi par excellence”. But, since this review was published, and I attempted to discuss several of the erroneous specifications and claims for the Mini3 on the AMB forum, AMB has banned me from their forums. So it’s a bit more difficult to praise a website that not only makes a lot of misleading and erroneous claims but censors even their factual discussion. AMB is not doing the DIY community any favors with either of these things.

MINI HEADPHONE AMPS: Some might be familiar with the Cmoy designed by Chu Moy and various similar “mint” amplifiers built into Altoids tins. A lot of portable audio gear runs from a battery voltage around 3 volts and has a tough time driving some high impedance headphones. Amps like the Cmoy typically run from 9 or 18 volts allowing them, in theory, to do a better job with such headphones. You can find several variations on eBay but its important to understand their limitations. A Cmoy can be worse than no amp at all in many applications.

SMALL & SIMPLE: It doesn’t get much more basic: Input, Output, Volume (with on/off switch), and a DC jack for charging. It’s small and easy to toss in your backpack. You plug in your source and headphones, turn it on, and listen.

AUDIBLE HISS & NOISE: I listened to the Mini3 with some full size headphones and didn’t notice any obvious problems. It was fairly quiet even with my SuperFi 5’s and dead quiet with full size cans.

VOLUME & GAIN: The Mini3 gets plenty loud with low impedance or high efficiency headphones. But if you listen to music with a wide dynamic range on low efficiency high impedance headphones it might clip off the peaks or not play loud enough. It has about 14.5 dB of gain which is on the high side for the more efficient headphones it’s best suited for. I prefer amps with a selectable low/high gain option. As a DIY project, if you build it yourself, you can lower the gain by reducing the value of two resistors (R4L and R4R) as needed.

BATTERY LIFE: Even with a brand new fully charged battery, my tests show you can only expect about 4 – 5 hours per charge from a typical rechargeable battery. This is substantially less than most portable players. As the battery ages, that number will only get worse. I’m not sure how AMB claims 10+ hours for this version as the highest capacity NiMh batteries can only bump the runtime to about 7 hours. The 3 channel design, high performance op amps, and rail splitter, combine to require a lot of power. It’s possible the “extended run” version gets closer to its 20+ hour rating but, for reasons I explain later, I don’t recommend it. See the Tech Section for more on all of these topics.

MEASUREMENT SUMMARY (revised 5/13): The Mini3 did a few things well but failed to meet several published specifications (see the start of the tech section where I specifically tried to recreate several of AMB’s test conditions). The good news included flat frequency response and it’s fairly quiet. But, mainly due to using an “active ground” that’s shared between the channels (a three channel design) it doesn’t like low impedance loads, has poor channel separation, excessive high frequency distortion and it’s prone to a particularly objectionable kind of intermodulation distortion between the channels. See the Tech Section and my article on Virtual Grounds for more details. The following table compares my measurements to the AMB provided specs, the $99 FiiO E7, and the $20 FiiO E5:

Measurement

Mini3 Result

AMB Spec

FiiO E7

FiiO E5

Frequency Response

+/- 0.1 dB Excellent

+0 -3dB

+/- 0.1 dB Excellent

+/- 0.1 dB Excellent

THD 1 Khz 150 Ohms

0.002% Excellent

0.001% 330 Ohms

0.003% Excellent

0.005% Excellent

THD 1 Khz 15 Ohms

0.017% Good

0.001% 33 Ohms

0.03% Good

0.012% Good

THD 20 hz 15 Ohms

0.01% Very Good

Not Specified

0.09% Good

0.6% Poor

THD 20 Khz 15 Ohms

0.45% Poor

Not Specified

0.06% Excellent

0.05% Excellent

IMD CCIF

0.043% Fair

Not Specified

0.03% Good

Not Measured

IMD SMPTE

0.009% Very Good

0.006%

0.008% Excellent

0.006% Excellent

Noise (ref 400 mV)

-94 dB Excellent

-93 dB

-91 dB Good

-86 dB Fair

Max Output 15 Ohms

104 mW Excellent

300 mW 33 Ohms

113 mW Excellent

108 mW Excellent

Max Output 150 Ohms

38 mW Fair

Not Specified

23 mW Fair

22 mW Fair

Output Impedance

0.9 Ohms Very Good

< 0.1 Ohms

0.13 Ohms Excellent

0.7 Ohms Very Good

Crosstalk 15 Ohms

40 dB Poor

88 dB

63 dB Very Good

46 dB Fair

Channel Balance Error

1.14 dB Fair

Not Specified

0.2 dB Excellent

Sample Problem

DIY PRIDE: It’s easy to think something you soldered together yourself sounds amazing. I’ve been there. DIY efforts are often labors-of-love. And, because of that, the subjective impressions are even more biased than usual (see: Dishonesty of Listening). It’s really easy to fall in love with something you created.

DIY DESIGNS: Some DIY’ers tinker with their own “one-of-a-kind” designs while others prefer to let someone else do the design work. The Mini3 design I tested has been around since 2007. It’s supposedly a mature design that’s been extensively discussed on forums and suitably optimized based on the iterations that came before it. It’s well documented including fairly complete specifications on the AMB website. Yet, even after all that refinement, it still has some significant problems. If a design that’s been around for several years has these sorts of problems, what does that say about a design someone throws together in their basement? Unless they have a way to properly measure how it really performs, they really don’t know what they’re getting. RMAA, as used by AMB and most DIYers, has lots of problems and issues I discuss here. Obviously, RMAA didn’t cut it with the Mini3. Several of the published RMAA measurements are wrong by a wide margin.

DIY MEASUREMENTS: The Mini3’s performance suggests RMAA and typical basic instruments are often not enough to verify audio gear is working as intended. At the beginning of the Tech Section there are several examples where the numbers claimed by AMB are literally impossible. First of all, a good audio designer should know what range of performance is realistic. If numbers are impossible, even in theory, they should suspect something is wrong with their measurements. And, if you can’t properly verify the performance, how do you know you got it right? And it’s been well proven sighted listening can be very misleading.

DIY vs COMMERCIAL: Commercial manufactures have far more resources to design and test their products. They also generally make sure what they’re shipping actually matches the advertised performance. So it’s not surprising the $20 FiiO E5 can beat or rival the $180 (assembled) DIY Mini3 in several areas. Being mass produced in China, of course, doesn’t hurt either. It’s much the same story with the Behringer UCA202 $29 DAC. But such products are neither unique nor offer much in the sense of accomplishment—like a DIY project can. But some guy in his garage or basement using a soundcard and RMAA will have a hard time, no matter what designer parts he uses, matching experienced engineers with $100,000+ R&D labs at commercial companies. It’s like some guy trying to build a new car in his shed. Does he really think he’s going to do a better job than Mercedes or even Volkswagen?

DIY VERDICT: It’s not fair to judge all DIY designs by the Mini3. DIY offers a level of satisfaction and customization that’s not possible with commercial products. For some, these things are the main priority. DIY is also more cost effective than it used to be with more options today for PC boards, enclosures, parts from Asia, etc.

IMPROVING DIY? In my opinion, the DIY community could benefit, and save a lot of money, with more solid engineering and less audiophile inspired “tweaks” including questionable design topologies. My 3 Channel article discusses how audiophiles sometimes end up going down the wrong path leading to products that measure, and likely even sound, worse. Those looking for an off-the-shelf design as a starting point for their project should ideally have reliable published specs to help them decide if a given project will meet their needs, is worth the time, expense, etc. You can’t just return a DIY product to the store if you end up not liking it. So DIYers should have as complete and accurate of information as possible before choosing a design to build.

ALTERNATIVES (revised 5/28): Being purely objective, the $20 FiiO E5 outperforms the Mini3 in several areas (see table above). It’s much smaller, has better battery life, is happier with low impedance loads, and you can buy two of them for the $40 cost of just the Mini3’s front and back panels. The FiiO E7 at $99 is a decent USB DAC and headphone amp along with 80 hours of battery life, bass EQ, and more. An excellent DIY sites worth checking out is:

DIY DECISIONS: I believe those choosing among various DIY designs deserve honest information before they invest lots of time and their own money. Several tube DIY headphone amps, for example likely offer much worse performance than the Mini3, but at least I’m not aware of any making the sorts of misleading objective claims that AMB makes for the Mini3. I strongly believe objective claims for things like power output, battery life, crosstalk, etc. should be reasonably accurate.

MINI3 vs CMOY (added 5/27): I just finished testing a dual battery classic Cmoy design based on a much cheaper op amp than the Mini3. It outperformed the Mini3, often by a significant margin, in nearly every test. See my Cmoy eBay Amp Review for more.

BOTTOM LINE: Does the Mini3 “rival amplifiers many times its size and cost” as claimed? In my opinion no. At $180 in pre-assembled form it’s not exactly cheap. And even as a DIY project if you buy the PCB, case, front/back panels, etc. you could easily get close to the price of several other options. But DIY isn’t always about being cheaper. The Mini3 stakes out an awkward middle ground. It doesn’t have enough output to be an ideal match with really power hungry cans. And it has significant distortion into low impedance loads rendering it a poor choice for many portable headphones. It might work reasonably well with some high impedance headphones if you’re OK with the limitations.

TECH SECTION:

AMB’s MEASUREMENTS: The “Specifications” section of the AMB website lists an impressive set of specs and measurements. Unfortunately, a lot of them are wrong and might mislead prospective DIY’ers considering the Mini3. Here are some examples where I tried to recreate the AMB test conditions:

CLAIMED POWER:300 mW @ 33 ohms vs REALITY:98 mW @ 33 ohms. AMB’s 300 mW requires 3.15 V RMS (V = SQRT(P*R) = SQRT(0.3*33) = 3.15). That’s 8.9 volts peak-to-peak. In reality, the output can only get within about 0.25 volts of each rail into 33 ohms per the AD8397 datasheet. There’s a 0.7 volt diode drop off the battery plus a 6.2 ohm output resistor which drops about another 1.6+ volts. So even with a freshly charged battery at 9 volts, you end up with only about 6.2 volts of swing at the output. That would be good for 145 mW into 33 ohms best case. But, especially with both channels working, the OPA690 virtual ground has a very hard time supporting the required current and its distortion rises rapidly. So, in reality, the Mini3 only measured 1.8 V RMS into 33 ohms or 98 mW at 1% THD. That’s only 1/3 of what AMB claims but represents 154 mA of total peak current which is very close to the 160 mA limit specified by TI for the OPA690.

300 mW RMS TAKE TWO: The AMB specs page says “These tests were run with the amplifier operating on battery power.” But, just to be fair, I also tried an external 15 volt DC power source. The Mini3’s 7812 regulator and 1N4001 diode limit the internal supply voltage to 11.3 volts under load regardless of the DC supply voltage connected. In theory, that’s good for about 3.2 V RMS of output. But into 33 ohms, the Mini3 still has a hard time because current, not voltage, is the problem. It hits 1% THD at 2.0 V RMS or 121 mW RMS. That’s still well under half what AMB claims. The problem here is 121 mW is 86 mA of peak current per channel or 172 mA total out of the OPA690.

300 mW RMS TAKE THREE (added 5/28): If you only drive one channel, and use the AC adapter, the Mini3 can in theory manage 3.2 V RMS into 33 ohms. This is 310 mW. I didn’t try to verify this, but I suspect it will hit 1% THD before it reaches 300 mW because of the relatively high peak current required. But, if you ignore the usual 1% THD limit, it might actually make 300 mW into 33 ohms with just one channel operating running into clipping on AC power.

CLAIMED POWER: 30 mW @ 330 ohms vs REALITY:18 mW Working backwards the claimed 30 mW is 3.15 V RMS. Where have we seen that number before? Perhaps someone at AMB only calculated the power from the battery voltage and neglected all the other losses in the circuit and never made any real measurements? The reality is 2.45 volts into 330 Ohms at 1% THD = 18 mW—about half the claimed power. This is a somewhat marginal number for many headphones.

CLAIMED BATTERY LIFE (revised 5/10): 10+ Hours vs REALITY: 4 or 5 hours. Typical NiMh 8.4 volt batteries are 180 – 200 mAH. These will yield about 4 – 5 hours run time in typical use. There are some 300 mAH “max” or 270 mAH “typical” 8.4 volt batteries and you might get 7 or so hours with one of these. If you look at a typical battery datasheet, you’ll see the 35 mA typical consumption of the Mini3 works out to a battery life of 4 – 5 hours.

CLAIMED THD: 0.001% @ 33 ohms vs REALITY:0.01% This one’s a bit tricky because, like so many RMAA results, there’s no mention of what level was used for the test. But at my standard listening level of 400 mV the THD+N was 0.01% into 33 ohms. The Mini3 just doesn’t like low impedance loads. The dScope graphs for 15 ohms are shown later and show distortion is relatively high at every output level and much worse at high frequencies.

CLAIMED STEREO CROSSTALK–88 dB vs REALITY:-46 dB: AMB quotes RMAA crosstalk into 33 ohms as 88 dB. The “shared ground channel” design of the Mini3 does horrible things to the crosstalk but here’s some very revealing math assuming 1 volt of output into AMB’s 33 ohms: AntiLog(-88/20) = 0.000004 volts (40 uV) of signal in the muted channel. Because of the isolated input and output grounds, all ground impedance in the Mini3 contributes to the crosstalk. So in the working channel the ground impedance forms a voltage divider with the 33 ohm load. To get the output in the muted channel below the required 40 uV (-88 dB) you have: 33/(1/0.000004 - 1) = 0.0013 ohms (1.3 mOhm)! This is an impossible number. Even just the output jack in the Mini3 has a 50 mOhm spec—38 times higher! Add in the copper PCB traces, ferrite bead, and most of all, the output impedance of the OPA690 ground amp, and you end up with about 150 mOhms total which gives somewhere around –46 dB. I suspect the –88 dB AMB measurement is with no load making it an entirely useless measurement. And because RMAA measures everything at once, that would also mean that all the other measurements are also with no load. The dScope graph for 15 and 150 ohms is shown later.

HIGH PERFORMANCE? Lots of claims are made for the Mini3’s performance. The High Performance Edition uses a relatively expensive ($6 – $7) Analog Devices AD8397 op amp in a modified Cmoy-like configuration. This op amp is notable for its unusually high peak current capability. AMB claims it can “drive low and high impedance headphones with authority” and has “very high slew rate, wide bandwidth, low output impedance and low distortion.”

EXTENDED RUN TIME VERSION: The longer battery life version uses the National dual LMH6643 and single LMH6642 op amps which draw less quiescent current. But they’re only rated at 75 mA of peak output current, and as you’ll see in the measurements, that just makes a serious problem much worse partly because of the 3 channel design. They’re also not designed for audio use, have relatively high distortion, higher noise, and way more bandwidth than is needed. I personally think they’re a poor choice for any audiophile application. I did not test the extended run version.

3 CHANNELS ARE BETTER? Several AMB designs use 3 channels. AMB makes all sorts of claims for the “3-channel active ground topology” of the Mini3 and some of their other designs. Here’s the impressive sounding description for the Mini3:

“The ground channel amplifier sources or sinks the return current from the transducers, which would otherwise have been dumped into signal ground or power supply ground. This shifts responsibility for the high current reactive load of the headphones from signal ground to the power supply rails, thus removing the primary source of signal ground contamination. The headphone transducers "see" active amplifiers on both sides, rather than an amplifier on one side and a capacitor bank of the power supply ground on the other. This results in lower output impedance, greater linearity and reduced stereo crosstalk.”

THE FACTS:

Greater Linearity? Nope! Active amplifiers have distortion and non-linearities. To have the headphone “see” two amplifiers instead of one increases distortion and decreases linearity with two active devices adding distortion instead of one. You essentially double the potential non-linearities.

Reduced Stereo Crosstalk? Nope! When you share a fake ground between the two channels, you dramatically increase the stereo crosstalk as shown in the measurements and the math is shown a few paragraphs above.

Lower Output Impedance? Nope! This would require the virtual ground channel to have negative output impedance. The reality is the virtual ground raises the output impedance.

FAKE GROUNDS (updated 5/13): The Mini3, in part, prompted me to write an entire article on virtual grounds. AMB chose a single battery design using a virtual ground and, in this case, it causes several problems—some likely audible.

UNSAFE SOUND: Some purists want as little as possible between them and their music even if it means no protection. The Mini3 is fully DC coupled and has full gain at DC with no capacitors to come between you and the music. That might sound great but it’s risky for your headphones—especially for a DIY design. If your source has DC offset or leakage (many do) the Mini3 will faithfully amplify the DC and send it onto the headphones. You can’t hear DC but even small amounts can damage headphones. And, if anything goes wrong with the Mini3, including even a bad solder connection, it’s likely your headphones could see LOTS of DC and be damaged. The OPA690 “virtual ground” is especially stressed in this design. It gets seriously hot when the amp is driven hard into low impedance headphones. If it fails, your headphones could easily be damaged. With op amps, if pretty much anything goes wrong, their outputs usually slam hard into a supply rail. And because of the virtual ground design, there’s more to go wrong with the Mini3 compared to a dual battery Cmoy or the self-contained and self-protected chip amp in the FiiO E7 and E5.

MY USUAL TESTS: It’s clear much of the Mini3’s published data at 33 and 330 ohms is, at best, optimistic. To more thoroughly evaluate the performance I made my usual measurements at 15 and 150 ohms consistent with the other headphone gear I’ve tested. For more on how I test, please see: Testing Methods

FREQUENCY RESPONSE: The Mini3 was impressively flat. The graph below shows both channels into a 150 ohm load (pink/red) and also a 15 ohm load (yellow/blue). You can see about 0.25 dB channel balance error at this volume setting (50%) and about a 0.5 dB drop into the 15 ohm load. The Mini3 also has a bit more HF roll off into a low impedance load perhaps due to the ferrite bead on the output. It’s about 1 dB down at 80 Khz. The insignificant LF roll off is likely the DC blocking in the dScope:

THD 15 OHMS: Using a more challenging load, there’s about 8 times more distortion and the highly audible 3rd harmonic is rather high at –75 dB and cause for some concern. Anything above -80 dB is considered potentially audible by many experts. Because the current required is well within the limits of the op amps, I suspect the 6 ohm series output resistor in the feedback loop might be partly to blame here:

THD RESIDUAL: The residual shows the harmonic components and some minor crossover distortion. The waveform is somewhat oddly shaped likely from the combined distortion products of the OPA690 trying to maintain the virtual ground:

THD 20 KHZ: One disadvantage of a “single stage” design like the Mini3 is the hard working output stage also has to provide gain. This is sort of analogous to why the best sports cars are not front wheel drive. It’s a compromise when the driving wheels also have to handle steering duty. With the Mini3 there’s less feedback available at high frequencies to correct the output stage distortion. And, perhaps combined with other problems (like that 6 ohm resistor previously mentioned and possible borderline instability), the result is a lot of high frequency distortion. It went from a reasonable 0.016% at 1 Khz to 0.45% at 20 Khz—nearly 30 times higher:

THD 20 HZ: At 20 hz the Mini3 is much happier but the nasty 3rd harmonic is still flirting with 80 dB:

IMD CCIF: Again, the Mini3 is not very happy at high frequencies into a 15 ohm load. The huge “mountain” reaching up to almost –50 dB around the 19 & 20 Khz tones indicates potentially objectionable odd-order IMD distortion products and is almost certainly audible under some conditions. And remember this is only at about 400 mV output, it gets much worse at higher levels:

SMPTE IMD: The Mini3 does much better here with nearly all distortion products below –90 dB:

OUTPUT IMPEDANCE: From the reference level of 400 mV into 150 ohms, the Mini3 produced 404 mV into 100K and 381 mV into 15 Ohms. This works out to an output impedance of 0.9 ohms. This is acceptably low but the 6 ohm series output resistor may stress the negative feedback loop under some conditions which is likely responsible for some of the high distortion seen above.

MAXIMUM OUTPUT & THD+N vs OUTPUT LEVEL: This graph kind of tells it all. Into 15 ohms the Mini3 is constrained by distortion that quickly hits 1% at only 1.25 V RMS of output and barely dips below 0.01% at all. 1.25 volts is 118 mA of peak current per channel and both channels are driven. So the OPA690 “ground channel” is having to source and sink 236 mA of peak current but is only rated for 160 - 190 mA. The ugly result is obvious below. Into 150 ohms the Mini3 is much happier with distortion under 0.002% over nearly all the output range up to well over 2 volts. Compared to the $99 FiiO E7 the Mini3 does worse at 15 ohms and better at 150 ohms with 104 mW and 38 mW respectively:

GROUND CHANNEL BLUES: Given the poor performance into 15 ohms I wondered how many of the problems noted above were due to AMB’s “virtual ground channel”. If the OPA690 is the limiting factor, it’s going to create an even bigger problem with real stereo music. In the above tests the right and left channels are reproducing exactly the same signal. With stereo music, however, the two channels each place different demands on the OPA690 but the OPA690’s distortion will be shared between the channels. So, put simply, each channel’s distortion adds to the other. This is especially bad because some of this “shared” distortion will not be harmonically related to the music in that channel and thus far more likely to be audible.

THD 15 OHMS ONE CHANNEL ONLY: Here’s just one channel driven to 1 volt of output into 15 ohms. The distortion is a moderate 0.03%:

THD 15 OHMS BOTH CHANNELS: Here’s the exact same measurement as above, but the other channel is now also operating. Notice the distortion goes from 0.03% to 0.32%—ten times higher. This makes it obvious the problem is the shared OPA690 “ground channel”. It’s just not up to properly supporting both channels into a low impedance load—even at only 1 volt of output you have the highly audible third harmonic hitting almost -40 dB. Listening to this signal, versus the one above, the added distortion is plainly audible using the dScope’s monitor feature:

INTERCHANNEL IMD: The above points to a serious IMD problem between the stereo channels due to the shared virtual ground. To test that theory here’s one channel driven by 1 Khz and the other channel (not shown) driven by 300 hz—both to 1 volt RMS. Note the OPA690 is operating “within spec” here as the total peak current is under 160 mA. The –54 dB spike at 300 hz is crosstalk from the other channel. The relative “forest” of other spikes are the combined IMD distortion mostly due to the shared OPA690. You can’t see it in a single still image, but the “forest” was constantly changing due to the way the 300 hz and 1000 hz signals overlapped their peak current requirements from the OPA690. This is a significant problem. A residual of –53 dB is 0.22% and much of it is not harmonically related making this sort of distortion much more likely to stand out and be audible. Indeed, using the dScope monitor, the 1 Khz signal sounded plainly cleaner when the other channel wasn’t in use. The difference wasn’t subtle:

CHANNEL SEPARATION: Given the above, you might expect poor crosstalk performance. Indeed into 15 ohms it was only about 40 dB which is fairly lo-fi. Into 150 ohms it was a respectable 61 dB. By comparison, the FiiO E7 is a huge 20 dB better with 60 dB into 15 ohms:

NOISE: The Mini3 can be impressively quiet--significantly better than the FiiO E7’s analog input for example. The 60 hz hum spike is from the Mini3’s PC board picking up stray AC fields. It’s nothing to worry about at only –110 dB but it does show the Mini3 is a bit more prone to EMI problems than some designs including even the lowly FiiO E5:

CHANNEL BALANCE: With the volume control near 9 O’clock I measured the worst-case channel balance error at 1.1 dB. This is just over the audible threshold of 1 dB and probably not a big deal but the FiiO E7 does considerably better at < 0.3 dB across the entire range:

GAIN: The Mini3 has about 14.4 dB of gain (unity gain is about 11 O’clock on the volume control). The op amp itself runs (assuming accurate resistor values) at 14.9 dB but there’s some slight attenuation in the input circuit. Into 15 ohms 210 mV RMS of input was enough to produce output clipping with the volume set to maximum:

PHASE: The Mini3, as expected, does very well on this test—perhaps too well. It’s considered good practice to restrict bandwidth for a variety of reasons but the Mini3 runs “wide open” to whatever bandwidth the AD8397 can manage:

SQUARE WAVE PERFORMANCE: The blue trace below is the output of the Mini3 into Sennheiser CX300 headphones representing a real-world reactive load. The red trace is the input signal from a waveform generator. The “overshoot” on the output could be a sign of marginal high frequency stability and the “squiggle” that follows is an oscillation with a period of about 250 nS or 4 Mhz. This, combined with the dramatically higher distortion seen earlier at 20 Khz, is a good indication the Mini3 may need more work to assure stability under all conditions:

SLEW RATE: If you look at the table in the graph below you’ll see the Mini3 managed about 0.64 volts of “slew” in 60 nS between points “1” and “2” on the blue curve. That’s about 10.7 volts/uS and is nearly 20 times faster than the Mini3 needs to be. The Mini3 only needs about 0.6 V/uS to reproduce any musical signal you’re ever likely to feed it. It’s been well demonstrated by Douglas Self and countless others 0.2 V/uS slew rate per volt RMS of output is more than enough. So even into a high impedance load on AC power, the Mini3 can only manage about 3 volts RMS. Hence it only needs a slew rate of 0.6 V/uS. Anything more is meaningless and may require design trade-offs that degrade the performance in other ways. You can also see the period of the ringing is about 250 nS. Here’s the result:

DESIGN COMMENTS: I was disappointed the Mini3 didn’t come close to several of AMB’s specs and claims. Here are some things AMB might want to consider:

Don’t Use A Ground Channel: The OPA690 ‘fake ground’ is the biggest problem with the Mini3. It adds a lot of distortion that could be audible. It also destroys the channel separation, is the main bottle neck limiting output power into lower impedances, and seriously decreases battery life. The OPA690 is nearly a $5 part plus another $3 for the TLE2426 and related parts. The $8 could buy another NiMh battery and a real bipolar power supply. A simple trickle charger could be used from a 24 volt DC wall transformer to charge it. There would be a huge increase in performance and longer battery life. And it removes a likely point of failure that could easily destroy a pair of headphones with unwanted DC. Dual batteries would greatly increase the output swing into high impedance loads making the Mini3 much more suitable for power hungry full size cans. This change would, however, require reviewing/updating some other aspects of the design.

Remove The 6 Ohm Output Resistor: This crude form of short circuit protection is significantly compromising the performance. It dramatically increases the feedback error signal and reduces maximum power when driving low impedance loads. I suspect it’s partly responsible for some of the high distortion levels. And, worse, with a reactive load (like headphones) the voltage drop across the resistor will be out of phase with the output of the AD8397. This creates a phase shift in the feedback loop potentially reducing stability (phase margin) and leading to possible oscillations—like the hint of 4 Mhz ringing I observed. If the AD8397 cannot survive a short circuit without the series resistor, there are other op amps that are short circuit protected and might offer better overall performance. Another option would be a more elaborate protection scheme. A final (low cost) compromise might to move the resistor outside the feedback loop—especially if you only plan to use higher impedance conventional headphones (not BA IEMs).

Improve High Frequency Stability: There’s enough evidence the Mini3 might be marginally stable to consider some improvements or at least investigate further. The AD8397 is a fast op amp and it demands very careful implementation. The AMB Mini3 History page describes having oscillation problems during development—I’m not surprised. The Mini3 is roughly 20 times faster than it needs to be. I would add a compensation capacitor to the feedback loop creating a dominant pole to greatly improve stability. AMB runs this AD8397 “wide open” for bandwidth when there’s no arguable benefit, and only risk, in doing so. I’d also look hard at the PCB layout, power supply decoupling, etc. It’s also highly possible the isolated input and output grounds are degrading stability.

Update the PCB Layout: I saw indications the Mini3 is more susceptible to external fields than it should be. This is almost always because of poor PCB layout and/or ground problems. There are also signs of marginal high frequency stability and this may also be a PCB layout problem (see above). Ground fills, as used on the Mini3 are cut up into “islands”. Ground currents are forced to take a much longer path around all the interruption compared to a real ground plane. So ground fills tend to work nothing like a real ground plane unless very careful attention is paid to all the ground current paths.

Remove PCB “Ground Strips”: Enclosures should never be used to carry signal ground currents. They should be grounded only at a single point using a nut and washer for a consistent low resistance connection. The Mini3, however, has long ground strips the length of both sides of the PCB that make contact with the “rails” inside the aluminum case (see photo above right). This creates multiple problems. Anodized aluminum is an excellent insulator, and even raw aluminum quickly forms an insulating layer of aluminum oxide when exposed to air. So any connection between the case and the ground tracks that slip into the “rails” is likely to be intermittent and rather random. That’s a recipe for intermittent noise and/or having ground paths change over the life of the amp. Having ground signal currents flowing through parts of the case may have contributed to the hum/EMI problem I observed. It’s really poor design practice—especially here where it forms “loops” from one side of the board to the other through the top and bottom halves of the case.

Add an Input Coupling Capacitor For DC Protection: A high quality input capacitor would not change the sound but it would provide much needed DC protection and help to prevent expensive headphone damage. Such capacitors have been proven transparent with measurements, blind listening tests and audio differencing.

Add RFI Protection: It’s considered good design practice to have an RFI filter on the input stage of audio op amp devices. RF (say from a cell phone) picked up by external cables can easily be demodulated at the AD8397’s input creating dc offset, noise and/or distortion. A simple capacitor to ground in each channel and a ferrite bead would be a huge improvement. Again DC can damage headphones.

Revise The Specs & Claims: Especially with a DIY project people expect honest information to base their project decision on. AMB should not claim an active ground channel “results in lower output impedance, greater linearity and reduced stereo crosstalk” when the opposite is true of all three. And they should make proper measurements (or find someone who can) and avoid publishing misleading specifications.

FINE PRINT: The Mini3 PCB was version 2.01 and built to the standard AMB design minus D4 which has no effect when the battery is installed. It should be consistent with those available from MisterX (Marshall Wyant), YBM Audio, or Wilmer Workshop. Except as noted, the Mini 3 was run from a freshly charged 8.4 volt NiMh battery for all the tests (consistent with AMB’s Specifications page). My instrumentation and methods are detailed here: Testing Methods

TECH SECTION SUMMARY: I wasn’t expecting perfection from a simple single op amp Cmoy design, but I was still relatively disappointed in the AMB Mini3. It didn’t come close to meeting many of its published specifications even when I tried to re-create AMB’s test conditions. And it has some potentially significant design problems. AMB claims it can “drive low and high impedance headphones with authority” but, in reality, it doesn’t much like low impedance loads even at typical levels of only 400 mV. With 80 ohm or greater headphones, it will likely work reasonably well as long as you’re OK with the short battery life, DC risk, potential stability issues, and it has enough voltage swing for your cans. But I strongly suspect the AD8397 op amp would perform better in more of a classic “Cmoy” design running from 2 batteries. That eliminates the need for the third “virtual ground” channel that causes so many performance issues with the Mini3. Dual batteries, however, create other potential issues—see my Virtual Ground article.

REAL GROUND FOLLOW UP (added 5/27): See my test of a classic 2 battery Cmoy which, despite using a much cheaper op amp, performed better than the Mini3 in nearly every test. It’s further evidence virtual grounds don’t work as advertised.

O2 DIY AMP FOLLOW UP (added 8/7): I have since designed a similar amp using 2 batteries and a conventional ground. Despite using much less expensive op amps, the O2 DIY Amp outperforms the Mini3 in every measurement. It can also suitably drive most any headphones, has longer battery life, battery protection, RF protection, and costs less. The O2 is open source hardware and the design is available to anyone.

D4 (the zener) does nothing when running from the correct battery. And it's considered bad practice, as I explain in my review, to use anything besides a single ground point to an enclosure. Doing so can let ground signal currents pass through the case which is a bad idea for many reasons. The ground strips should be removed from the PCB--they're likely part of the problem, not the solution.

D4 is in parallel with the battery according to the schematic. A 12V (breakdown voltage) Zener diode is pretty much an open circuit when the voltage across it is about 9V from the battery and not 12V or higher.

Anyway, an issue with the supply voltage probably shouldn't cause the kinds of issues observed for the device.

Very interesting article, thanks. Judging by their RMAA results*, I had expected the "high performance" variant to be overall "excellent". I must admit, though, that most of your tech section goes over my head.

How did you define those measurement summary quality attributes (excellent, very good, etc.)? At what point do you think become the differences humanly audible under ideal conditions (i.e., could a human hear a difference between excellent and very good, good or fair attributes)?

* http://www.ibiblio.org/tkan/audio/mini3.html

There's a small error in one sentence:"and leading to possible to [sic] oscillations"(No need to publish this part.)

Good point Yoga Flame, but not the case here. You would have to remove the battery to get dangerous voltages and it's not removable with the Mini3 in its enclosure. So the battery is really part of the circuit.

The OPA690 was working as well as can be expected for an op amp not designed for audio use. And, when op amps are damaged, 99.99% of the time they slam into a rail and stay there. I specifically verified the OPA690 wasn't doing anything weird because that was my first suspicion as to what might be wrong.

The fact is, TI specs the OPA690 at 160 mA of peak current and it's progressively less happy the more current you ask it to source and sink. Driving both channels into 33 ohms at the 300 mW AMB claims on their website would require a peak current of 135 mA per channel forcing the OPA690 to source and sink 270 mA which is far beyond its ability.

The OPA690's specified THD is relatively high as it's not designed for audio use. Between that and the peak current problem it's easy to see how it contributed to the measurements I obtained.

Hey Francis. Some wonder if I'll ever learn but I'd rather help people make more informed decisions even if I'm not very popular among some circles. I refuse to drink the Kool-Aid and go along with hype that's just not true.

Google's Blogger service has finally (hopefully) recovered from a massive outage lasting about 30 hours and some comments have been lost and others ended up being re-classified. If you posted comments in the last few days and don't see them here, that's why.

To the comment from May 9th asking about the "Excellent, Very Good, etc." categories, I'm working on an article covering that very topic--correlating measurements with what people can typically hear.

You must have put some *serious* time into this review, but why? Amb diy is more about looks like than SQ. Other engineers have tried to explain the truth and they jamb fingers in ears and chant. Even Ti Kan ignores the truth. They dont care about real engineering and would rather discuss wood finishes, volume knobs and other BS. How about reviwing a firestone, one of the ebay faves like a little dot (perhaps a tube version?), or the fiio e9? Your reviews are awesome but you should stick to stuff smart people actually would consider.

Thank you! Now I understand why MisterX is not recommending Mini3 for a low impendence/IEM headphones. Obviously he can hear the difference! The difference in objective measurements and RMAA published results is really staggering! I am a bit disappointed that AMB did not validate the RMAA data. Unfortunately Audio industry in general is not really willing to educate their consumers that good audio comes from a properly engineered audio devices and properly engineered audio devices are tested and measured all the way from the design to the implementation. Obviously listening evaluation is mandatory, but it is just one of the tests any audio device must pass. It would be nice to have a vendor independent lab where consumers could send audio devices for a technical evaluation with results published on internet. Yes, I would pay to test my gear! Until we have that lab please continue ;-)

Thanks for added comments. To "anon" at 4:18pm, I tested the Mini3 because it seems relatively popular, and the AMB website is among the best DIY sites I had come across. Like many, I assumed their published specs were close to accurate. Had they been closer, the Mini3 would be a much more impressive amp than it really is.

To "anon" at 6:54pm, thanks. I agree the audio industry gets away with a lot because there's so little proper independent testing being done. And I agree listening is also important. There are plenty listening and publishing their subjective opinions but we need more making the right measurements.

While Google was having their problems, I received several private messages asking about virtual grounds and 3 channel designs. Apparently a lot of people think they're a lot better than they really are. So I've added an article on that topic.

I didn't read the whole thread at AMB very carefully, but didn't AMB mention that the 300mW into 33 ohms (with some 9V battery at 10V) was for one channel only?

If the measurement was taken with only one channel with the other given nothing, then the active ground OPA690 would be stressed half as much, and you might not hit the current limit of that device. 300 mW at 33 ohms requires 135 mA peak (under 160 mA specified by OPA690), unless I'm too sleepy to do rudimentary calculations right now.

Mike, you are partly correct. As I said under "CLAIMED POWER: 300 mW" in the Tech Section above you can, in theory at least, get as much as 145 mW with only one channel driven. That's better than 98 mW but still less than half AMB's claim. And even if you increase the battery voltage to something that's highly unrealistic you still can't get 300 mW.

In the AMB thread I ran the numbers several different ways backwards and forwards (literally). It's really fairly simple and black and white. The Mini3 cannot manage 300 mW even with just one channel driven while running from the specified battery.

And who listens to their amp with just one channel operating? If one channel was a condition of the tests, AMB should have specified that.

Because AMB's 330 ohm and 33 ohm power values assume the exact same highly optimistic output voltage, it's my best guess Ti Kan simply calculated, rather than measured, what he thought the output power should be. And he made some fairly significant errors in doing so.

The sad part is: Instead of just correcting the obvious errors, Ti Kan chose to shoot the messenger.

Even after reading your review of the Mini^3, I'm still quite convinced that the Mini^3 you are using is somewhat defective (in one way or another). So to give us readers the benefit of the doubt, perhaps you can borrow a fully functional Mini^3 and run the entire test again? I suppose there must be a Mini^3 "fan" out there who will gladly volunteer for this.

But this time, please conform within the 33-300 ohm specification of the Mini^3 and do the test with the volume pot at maximum position. Again, this is for the benefit of the doubt and for people to know that you are really objective in your reviews.

Entaro, if you haven't already, please read my newer article about Subjective vs Objective audio. With all due respect, you are "convinced" my Mini3's defective by what evidence? Have you made your own measurements? You trust Ti Kan's numbers more than mine? Or you simply don't want to believe the Mini3 is that flawed?

Ti Kan has not disputed any of my measurements with any engineering facts. If my Mini3 was really defective, it would be trivial for him to do so. Instead, he's waved his arms, blocked me from his forums, attacked me personally, and said I "over drove" it.

This is completely wrong. Most of my tests are at only 400 mV (0.4 volts). That's a typical listening level for many headphones. I also show a graph of the distortion vs output level and into low impedances the distortion is relatively high at all levels. I also point out where my tests are within Ti Kan's claimed limits for the OPA690 virtual ground. So Ti Kan's "he over drove it" claim, and his other claims that are equally misleading, are what you should be questioning.

I made several measurements at AMB's specified 33 and 330 ohms, and I also provided the engineering analysis, and math, that supports my numbers. I have also shown, several times now on the forums, how some of Ti Kan's numbers are literally impossible to obtain on any Mini3. Even a Mini3 plated in gold, frozen to cryogenic temperatures, and blessed by the Pope, can't do what Ti Kan claims.

I was linked to this blog by Audio Designline. Very detailed effort on NwAvGuy's part. Appreciate you taking the time, trouble, effort to conduct and post your findings, thoughts and then even follow up with all the posters here.

People should learn to stop flaming. If you like the sound of a product, then buy it. Buyers/fans of the product should do themselves a favour and have the manufacturer address the claims made here about inaccurate specifications. I'll still buy a Mercedes SLK350 even if it cannot make 200mph but I won't be happy if Mercedes claimed it did.

As a follow up, I added a comment in the review it's possible, at least in theory, to get 300 mW running on AC power with only one channel driven. I don't know what the THD would measure (it may well be over 1%) but at least it's not impossible.

I also want to be clear even 100 mW into 33 ohms is fine for 99% of applications. The much greater concern are the high levels of distortion due to the virtual ground--especially the interchannel IMD. In some cases, even at moderate power levels, this distortion could be audible. And, regardless, AMB's claims the third channel provides greater linearity, improved crosstalk and reduced output impedance are simply false and misleading.

I've also received a few comments asking me about specific posts by Ti Kan at his AMB website. Most of the questions I know about I've already answered in the article and comments above, or in my 3 Channel Article and comments. I have also revised this review multiple times based on feedback from others and trying to be as fair and objective as possible.

If you can't find your question addressed somewhere already, I might suggest asking Ti why he has blocked my IP? I'm happy to participate directly in the discussions there if I'm allowed to do so.

"EDIT: I just fully charged a 7-cell Accupower and an 8-cell Maha Powerex on my smart fast charger, and the voltage across the battery terminals was 10.4V and 11.9V, respectively. In retrospect, I think my measurements may have been done with the 8-cell. That was over 4 years ago and my memory is a bit fuzzy on that now."

If this is true (an 11V output max) then AMB's specs are entirely plausible in terms of power output. We're looking at roughly 2.1v p-p voltage loss . . .

Of course, that's assuming you have a really nice battery. Your results are not the "ideal" which is great, but saying AMB is a liar when he's been rather forthcoming (especially after you were banned) isn't very becoming.

First of all, the 300 mW issue is just one of at least a half dozen areas where AMB has published misleading or just plain wrong information.

Second, 8 cell batteries are very rare as they can easily damage devices designed for 9 volt batteries. The AMB parts list specified a 7 cell battery and that's what ships with assembled Mini3's. Even with an 8 cell battery I doubt the Mini3 can produce 300 mW into 33 ohms even with just one channel operating.

Third, as I show in the Tech Section of the review, even 12 volts only results in 310 mW into 300 Ohms. So anything under about 11.9 volts is going to fall short. Even your 11 volts doesn't cut it.

Fourth, I have 4 different NiMh 7 cell "9 volt" batteries and the best of them is 10.15 volts fresh off the charger. not the 10.4 volts claimed.

Fifth, That 10.15 volts drops immediately as soon as you apply a load. This is due to the battery's internal resistance and also the rapid depletion of the surface charge. When you drive the Mini3 to full power output into 33 ohms, even with just one channel operating, the battery voltage falls (more or less immediately) to about 9.5 volts--exactly what all the 7 cell datasheets specify. And even that only lasts a few minutes.

I wouldn't know how "forthcoming" Ti Kan has been as he's banned my IP from even viewing the AMB website. I'm blocked out entirely. Everyone should be asking him, and themselves, why?

For those new to this discussion, I suggest checking out the following articles as they all demonstrate what's wrong with the Mini3 and it goes well beyond failing to produce 300 mW into 33 ohms as AMB claims:

"First of all, the 300 mW issue is just one of at least a half dozen areas where AMB has published misleading or just plain wrong information."

I wouldn't exactly call you honest or forthcoming either. You specifically tested a load the Mini^3 was never meant for, haven't retained consistency in all tests (see open pot debate for crosstalk), and have mangled many things Ti Kan have said.

Example: The 3 channel topology debate. Ti Kan argues

"He mentioned (and drew diagrams) to show that the output impedance of the active ground channel amplifier is "100x" that of "real ground". While he states the benefits of star-ground technique, he fails to mention that "real ground" on a conventional amplifier is the midpoint between the power supply's capacitors whose internal impedance also contribute to the overall ground impedance. For example, if you look at the datasheet of the Nichicon HE low ESR capacitors you'll find the impedance of the capacitors are typically many tens of milliohms. So much for that "1 milliohm" real ground in his diagram. "Ground" is not some magical black hole. It's made of real parts and real wires and has finite impedance. Any time current flows through that impedance, a voltage will be developed which will show up as "ground bounce". In a conventional amp that real ground is also the zero reference of all voltages even in the low-current portions of the circuit. If the ground potential is wiggling as a result of the output current returning to ground, then the whole circuit is on "shaky ground". Again, star-grounding technique does help, but will not totally eliminate it because we don't have superconductors and zero-impedance capacitors.

The active ground amp must have very low output impedance in order to reduce a similar ground bounce. But in this case, at least the ground bounce is only on the active ground, not on the whole circuit. One could design an amplifier whose output impedance is lower than a real ground's total impedance. This is true for the M³ and β22, the latter having an output impedance is the low-milliohms (the wires actually dominate the total impedance, not the amplifier). The Mini³'s output impedance is not quite as low because its opamp output stage is not as stout, but it has no real ground anyway, given a single battery as the power supply. Its design is the best that it could be, given the constraints of size and power source."

Are you saying that the capacitors don't increase impedance of the real ground? Moreover, have you fully calculated a real ground in practice in comparison to say the M^3's ground? Have you developed a real ground that would work to prove your point? Or are you pulling theoretical numbers without actually consulting parts documentation?

"Second, 8 cell batteries are very rare as they can easily damage devices designed for 9 volt batteries. The AMB parts list specified a 7 cell battery and that's what ships with assembled Mini3's. Even with an 8 cell battery I doubt the Mini3 can produce 300 mW into 33 ohms even with just one channel operating"

If you had looked at the measurements part of the AMB site under Mini^3 - we can see he got 10.9V P-P at least on a test with the external adapter -- MEASURED. We can argue all day that his absolute maximums are optimistic, but let's be realistic. With the external adapter one channel driven can easily reach the ~300mw he claimed for a single channel easy. The only question here is could he have had a particularly strong battery and lucked out on those measurements? Probably, and if you said he should lower them to be more realistic (for battery at least) then great. Instead you went in calling everything he claimed horse crap.

Please tell me where I've been dishonest or not factual? I've presented the measurements, the science, the theory, provided references, etc. And, in response, Ti Kan, and others, have presented weak hand waving sorts of arguments that go against the basic laws of electronics or have nothing to do with the issues being disputed. They're bugs on the windshield of the car being tested.

"You specifically tested a load the Mini^3 was never meant for"

Not true. The AMB website, and Ti Kan, have said several times the Mini3 is intended for low impedance loads including very popular 16 ohm headphones. I also tested at the exact same 33 ohms Ti Kan used for his measurements. So your statement is simply false.

"you'll find the impedance of the capacitors are typically many tens of milliohms"

And that has nothing to do with what we're discussing. The impedance of the power supply capacitors, contrary to Ti Kan's either misguided or misleading claims, has nothing to do with justifying a 3 channel amplifier. I explain this, in detail, in the 3 Channel Designs article.

"star-grounding technique does help, but will not totally eliminate it because we don't have superconductors and zero-impedance capacitors."

This is also false. Star grounding is the preferred way of doing it by the best amplifier designers in the world. Please check with Doug Self, Bob Cordell, etc. on how star grounds work. When properly implemented everything is referenced to the SAME point. The impedance of the capacitors is a non-issue. Their impedance does NOT degrade the effectiveness of the star ground.

Ti Kan is trying to solve a problem that does not exist. And his cure, as I've shown, is far worse than any problem he's identified.

"Have you developed a real ground that would work to prove your point?"

I don't have to. There are countless real ground designs that already have. I tested a real ground Cmoy Amp and have shown it has vastly less crosstalk, lower distortion and doesn't suffer from the serious interchannel IMD problems the Mini3 has. It uses a $0.80 op amp instead of the $8 one in the Mini3. Also, in the 3 Channel Designs article I show the results of an experiment that compares a virtual ground to a real ground. In both cases, the real ground is clearly superior.

"With the external adapter one channel driven can easily reach the ~300mw he claimed for a single channel easy. The only question here is could he have had a particularly strong battery and lucked out on those measurements?"

I've not disputed 300 mW with one channel driven using the AC adapter. But you can't somehow magically be "lucky", overcome the laws of electricity, and achieve the same thing from a battery. I've shown why countless times. You're beating a very dead horse and not providing any factual evidence to the contrary.

It seems that Anonymous on June 6 has a bone to grind with the NwAvGuy. I'm not a hardcore objectivist, all I care about is sound that I like.

Having said that, I won't buy a headphone amp that has high distortion and picks up noise (EMI) easily. I usually run my amp next to my laptop so it matters.

LOL normal listening levels with never take me to 300mW. More LOL - I have to buy fancy 9V batteries just so that it meets spec? More LOL - why argue about wonders of Ti Kan's 3 channel ground scheme when that thing distorts so much?

Really, some people just like to argue for the sake of it. NwAvGuy runs through the math fully, unlike these detractors who runs vague data by you and claim it's science.

Ti Kan never tested a 15 ohm load -- you're saying he said low impedance loads were okay and therefore it's a perfect test subject. Then, complain when the measurements aren't astounding (even though at the levels you pushed them to would deafen anyone).

Please explain how that isn't disingenuous? Furthermore, the crosstalk issue. Various other critical reviewers that perform measurements do full open potentiometers for crosstalk. It's true for about any potentiometer.

"Not true. The AMB website, and Ti Kan, have said several times the Mini3 is intended for low impedance loads including very popular 16 ohm headphones. I also tested at the exact same 33 ohms Ti Kan used for his measurements. So your statement is simply false."

Where on AMB's site did he give specs for 16ohm loads? Oh right -- nowhere.

"And that has nothing to do with what we're discussing. The impedance of the power supply capacitors, contrary to Ti Kan's either misguided or misleading claims, has nothing to do with justifying a 3 channel amplifier. I explain this, in detail, in the 3 Channel Designs article."

You explain what you think more than anything else. I don't see anything tested in practice.

"This is also false. Star grounding is the preferred way of doing it by the best amplifier designers in the world. Please check with Doug Self, Bob Cordell, etc. on how star grounds work. When properly implemented everything is referenced to the SAME point. The impedance of the capacitors is a non-issue. Their impedance does NOT degrade the effectiveness of the star ground."

Actually, they do diminish the effectiveness -- just not enough to be detrimental. In practice Ti Kan's may have a lower output impedance, the crux here is whether it's justifiable. In a portable design such as the Mini^3 a virtual ground or active ground is going to be needed due to layout (really that simple). I don't think the larger designs need it though, but that's not really the point here.

“Ti Kan never tested a 15 ohm load -- you're saying he said low impedance loads were okay and therefore it's a perfect test subject. Then, complain when the measurements aren't astounding (even though at the levels you pushed them to would deafen anyone).” and ”Where on AMB's site did he give specs for 16ohm loads? Oh right -- nowhere.”

Ti Kan, on his own forum, said 16 ohm loads are “fine” with the Mini3. And the website says the Mini3 is suitable for “low impedance” loads. Most “low impedance” headphones intended for use with portable gear, like the battery powered Mini3, are 16 ohms.

I also ran tests at the same 33 ohms Ti Kan used. The results are in the review.

As for “levels that would deafen anyone”, there are low impedance power hungry headphones the Mini3 will fall on its face trying to drive properly. Check out the Audeze LCD-2 or any of the planar models from HiFiMan. They have impedances in the 20 – 50 ohm range and low sensitivities. The Mini3 will struggle to drive any of these headphones at suitably low distortion. If you don’t believe that, I’ll be happy to provide the math.

Furthermore, the crosstalk issue. Various other critical reviewers that perform measurements do full open potentiometers for crosstalk. It's true for about any potentiometer.

I’ve addressed this multiple times. You don’t have to believe me, but the crosstalk within the potentiometer is insignificant compared to the crosstalk created by the virtual ground. So it doesn’t matter where the volume control is set, the Mini3 will still have poor crosstalk driving a typical load. It’s literally like claiming a bug on the windshield slows a car down.

”You explain what you think more than anything else. I don't see anything tested in practice.”

Really? Nothing tested in practice? I’ve shown detailed tests for the Mini3, tests of a real vs virtual ground in the 3 Channel Article, and I’ve shown the tests for a real ground $39 Cmoy Amp that blows away the Mini3 in many areas. So what exactly have I “not tested in practice”? Where are your test results?

The fact is, those who don’t agree with my results such as “anonymous” above, have failed to present any hard numbers, or other real evidence, of where I’ve made any mistakes. I’ll be happy to correct my review if someone can show where it’s wrong. For those who just have a hard time accepting reality, I strong suggest reading (or re-reading) my Subjective vs Objective article.

There are 31 comments for this article and 36 more in the follow up 3 Channel Article. Most of the negative comments are anonymous but seem to be the same person or two beating the same dead horse. Similar comments just keep coming in.

It's like an average person arguing the earth is flat. They don't have any real proof it's flat, but it sure seems that way to them. So they turn to discrediting the scientist claiming otherwise. They don't understand (or don't want to understand) the science well enough to believe his arguments. Their eyes tell them the earth is flat, therefore it has to be flat. Anyone who claims otherwise is obviously wrong. I cover this, in detail, in the Subjective vs Objective article.

My critics claim I had a defective Mini3 (I checked and didn't), the missing diode was a problem (it wasn't), not using full volume ruined all the measurements (it didn't), Ti Kan must have "gotten lucky" with his measurements so I should not dispute them (?!), using the same tests I use for every other device somehow wasn't fair, capacitor impedance ruins real grounds (it doesn't), etc.

I've spent a lot of time addressing the criticisms. I've taken more measurements, done more research, shown lots of math, discussed the same points in multiple open forums (others have agreed with my assessment), written an entire 3 channel amplifier article, tested a 2 channel real ground Cmoy (now with gain), and revised my articles several times with updated and additional information.

It's become obvious no matter what I say or do, the same person(s) will keep insisting the earth is flat and bugs on the windshield ruined my test of the car. I no longer have the Mini3 so I can't run further tests on it. And it's not productive use of my time or space in this blog to keep arguing the same tired, and mostly irrelevant, points. Unless my critics can present solid objective evidence of where I've made a mistake, please go find another cause--perhaps Judgment Day December 2012?

I'm about 99% certain that either your findings were wrong, your amp is shot, or there's such a critical flaw in RMAA it might as well be tossed out all together. I find it hard to believe the flaw would be wrong at over 20dB and nearly the same result in ten tests. For all we know you plugged/unplugged the amp while it's running and damaged the ground opamp (you do realize that can happen in this design right?).

I didn't publish your last message as it was almost entirely another lengthy re-hash of the same points already discussed ad nauseum. I can't edit posts via Google's Blogger so it's all or nothing.

The 60 dB crosstalk I measured is consistent with other devices and what could be expected. If the ground amp, PC board traces, and output jack have a combined total impedance of only 0.15 ohms you get 60 dB crosstalk with a 150 ohm load even if the rest of the Mini3, including the volume control, is perfect. This is discussed in the 3 Channel article.

It's reasonable to believe the ground system in the Mini3 has 0.15 Ohms of total impedance. Your 80 dB number into 150 ohms requires a total of only 0.015 Ohms which is much harder to believe as even the SG1-3533NG jack used has a contact resistance spec of 0.050 Ohms--over 3 times higher--all by itself!

As for RMAA, it's crucial the output be loaded with the correct impedance (150 ohms) for the test. How did you load your Mini3 for testing? Using a typical "Y" cable will degrade the crosstalk well below 80 dB all by itself. I use a custom test harness.

If you conduct the test with RMAA with no load, as most people do because they lack test loads, I might believe 80 dB as that takes the ground amp and other ground impedance out of the picture. But that's a worthless test as the amp is useless without driving a realistic load.

The math for calculating crosstalk related to ground impedance is basic. It's something any headphone amp designer should already know--including Ti Kan. Ti claims 88 dB with a 33 ohm load. For one channel to be 88 dB down from the other, with a 1 volt test level, the output of the non-driven channel has to be below:

AntiLog(-88/20) = 0.000004 volts (40 uV)

1 volt into 33 ohms creates a resistor divider with the shared output impedance. So for the first channel to create less than the above voltage in the other channel, the output impedance would have to be below:

Zout = Zload / (Vin/Vout - 1)

33/(1/0.000004 - 1) = 0.0013 ohms (1.3 mOhm!)

Datasheets for 3.5mm jacks list the contact resistance around 40 times higher at 0.050 - 0.100 ohms. So Ti Kan is trying to say his output jack, PCB ground traces, and worst of all, OPA690 ground amp, have a total combined impedance of only 0.0013 ohms? I don't think so. Not even close. The 50 mOhms of the output jack alone would degrade the crosstalk to 56 dB. The OPA690 very likely makes an even greater contribution degrading it further. So somewhere between the 40 - 60 dB I measured is entirely realistic.

Above is yet more hard proof how wildly optimistic some of Ti Kan's performance claims are. I've provided similar math for other optimistic numbers he's claimed. I know some would like to believe the laws of math and electricity somehow don't apply to audio gear, but they do.

As for the ground amp, or anything else, in the Mini3 being damaged, I didn't see any evidence of that. The behavior I observed can be explained by the datasheets for the two op amps and their implementation. Damaged op amps nearly always just slam into a supply rail or have other blatantly obvious problems. It is hard to imagine the Mini3 would measure as well as it did if either op amp was damaged.

I'm careful not to plug/unplug connections with devices powered up. But if simply forgetting to turn your Mini3 off before you plug/unplug headphones causes serious damage, that's an extremely poor design. That's almost certain to happen, at least accidentally, in the life of any headphone amp. If it can damage the Mini3, that's yet another strike against it.

I've shown, yet again, how (even under the best of conditions) Ti Kan's claims for the Mini3 are simply false. Anyone who understands dB, ohms law, and resistor dividers can verify that.

This blog limits comments to 4096 characters and I could not include a final point to the crosstalk explanation above. The Mini3 is at a further disadvantage for crosstalk (and shared distortion) because of its ground scheme.

In a conventional 2 channel amp with a star ground, some of the impedances in the ground system (such as the internal impedance of the power supply filter capacitors) are largely corrected for by the amp's feedback. This is explained in the 3 Channel article and is similar to how common mode rejection works. This is why a conventionally grounded headphone amp can produce crosstalk better than 80 dB into 150 ohms. The feedback loop of each channel is correcting for some of the shared ground impedance.

In the Mini3, however, Ti Kan chose to isolate the output ground from the input ground. So the inputs of the AD8397 audio amps are not referenced to the crosstalk (and distortion) error signals created at the virtual ground output of the OPA690. The same is true for the Mini3's "ground fill" PCB ground scheme. What an amp can't "see" it can't correct for. So the output of the Mini3, when it comes to crosstalk, is largely at the mercy of the total ground impedance. And that measures about 150 mOhms.

Please read this fully, as I want to make sure I've made no mistakes and relay my thoughts entirely.

"As for RMAA, it's crucial the output be loaded with the correct impedance (150 ohms) for the test. How did you load your Mini3 for testing? Using a typical "Y" cable will degrade the crosstalk well below 80 dB all by itself. I use a custom test harness."

I used a total of three separate wires, used heat shrink, on the left/right channel added 1/4 watt metalfilm 150 ohm resistors, added nothing to ground. Used a multimeter to confirm resistance, continuity, etc. I then used an external USB SBL! for output from my PC to the Mini^3, then the dummy load between the Mini^3 and the line-in. I then made sure nothing was clipping as I ran the mini^3 at half pot open and full pot open. I ran five tests of each. It was consistently between -79 and -81dB.

So, either I did something wrong and need to fix my dummy load (and I apologize if I did -- maybe you can post a tutorial on this?), RMAA is very wrong, or something is wrong with the Mini^3 you tested, or one of the parties made a measurement error.

"I know some would like to believe the laws of math and electricity somehow don't apply to audio gear, but they do."

That's the thing, I do believe in numbers, but I also believe in objective evidence. Ti and others provided links to batteries you said didn't exist. While his performance would drop FAST he measured it and has photographic evidence on an oscilloscope (read -- photographic showing P-P voltage). He mentioned after you were banned he may have used an even larger cell battery. While deceptive, I doubt it was intentional. He also has RMAA numbers with load, I see no reason why he would lie about the results. Whether is load was built right is another question to examine.

The fact that both of your numbers are so different on crosstalk more than anything else is raising very large flags in my head. Either RMAA needs to be blown out of all amplifier tests because it's so bad it misled AMB's design process, or your Mini3 was shot . . . or someone made a large mistake in measuring which is possible too.

"As for the ground amp, or anything else, in the Mini3 being damaged, I didn't see any evidence of that. The behavior I observed can be explained by the datasheets for the two op amps and their implementation."

As someone that had a ground opamp fail over time (slowly degraded, noise slowly started creeping, crosstalk started becoming overwhelming, and finally distortion heavily kicked in) on a Mini^3 I must say it can happen.

As for the unexpected pulling of the plug, I agree -- that is a more than valid complaint that would make me almost completely rule out the Mini3 as a portable (that's probably what started mine to its death). I consider it a portable desktop for when I get to hotels and stuff, but that's me. If you gave it a failing grade on that premise I would agree 100%. The numbers you're giving us though aren't clicking for me, Ti-Kan, and a few others in practice. I want to find the discrepancy. If I use a dummyload you okay and get numbers like yours my Mini^3 is being sold off and I'll just as fast buy an E5, Cute Beyond, or similar. Hell, maybe I'll build a wallwart cmoy (did once in the past).

Hopefully you can see I'm not trying to be a raging jerk for the sake of it. I'm a concerned owner that's trying to verify what both sides are saying to make an educated decision on what to do with the amp in my possession.

I've shown where Ti Kan's 88 dB spec is literally impossible. Even NASA couldn't pull off 1.3 milliohms. Measurements are not required to show his numbers are seriously wrong. They're also not required to blow your 80 dB number out of the water. Into 150 ohms at 1 volt the math is:

AntiLog(-80/20) = 0.00001 volts (100 uV)

150/(1/0.000005 - 1) = 0.015 ohms (15 mOhms)

The best case output impedance for a perfectly implemented OPA690 at 1 Khz is ~20 mOhms. Add at least 10 mOhm for PCB copper, another 20 mOhms for the ferrite bead at 1 Khz, 25 mOhms for a brand new $0.50 CUI tin plated jack, 25 mOhms for a brand new well made 3.5 mm plug, and you get:

20+10+20+25+25 = 100 mOhms (0.1 ohms)

20*Log(.1/(150+.1)) = -63 dB

So a reasonable best case cross talk number is -63 dB and I measured -60 dB with a $10,000 calibrated dScope while you're measuring -80 dB with a Soundblaster and freeware unsupported buggy software. Seriously, which number is more realistic?

I'm not sure if you read my RMAA article but there could be a few different problems with RMAA. But just because RMAA might get crosstalk wrong, doesn't mean it's entirely useless.

Crosstalk measurements are very challenging because of the extremely low levels involved. This is made worse by RMAA's lack of absolute levels and soundcard ADC scaling/limitations. At a calibration level of 500 mV RMS (which is quite loud) the muted channel during the crosstalk measurements would be at only 20 micro volts (50 uV) for Ti Kan's 88 dB figure. That's challenging territory for PC sound hardware.

One issue is how the signal is scaled before it gets to the ADC. The full dynamic range of the ADC might not be used. In the microvolt range you have only a few effective bits of resolution to work with. Lots of things can degrade the linearity, and accuracy, of measurements that low.

The dScope uses stepped attenuators with precision resistors to scale the input signal so it's usually within 2 dB of full scale of the instrumentation grade 24 bit ADCs. And it does so independently for each channel. So in a crosstalk measurement the input gain is optimized for each channel. This gives the dScope maximum resolution in the microvolt range.

RMAA was designed to mainly test soundcards. And I wonder if the developer sort of "worked backwards" from a well respected soundcard's specs allowing someone measuring that soundcard to get the expected results. If that's the case, it would explain why RMAA gets some things wrong--it's not calibrated at all.

RMAA might also, for example, use averaging to decrease noise and show more impressive numbers. But that would corrupt trying to measure anything else.

It might be an interesting exercise to explore RMAA's crosstalk measurements more. But that brings up yet another RMAA issue--using different PC sound hardware. I don't have your soundcard which might be the main problem.

I do think RMAA misleads a lot of people. I explain that in detail in the RMAA article. That's why I invested in a dScope. See Testing Methods

As for defending Ti Kan's measurements, I don't know what else to say. I've shown math nobody has disputed that proves how unrealistic some of his numbers are. From what I've seen, instead of admitting his car can't do 300 MPH, he's mostly made excuses, used false "psuedoscience", and/or avoided the real issues. He seems be trying, against all odds, to keep a myth alive.

The Mini^3 is producing almost near best case measurements into 150 ohms, but Ti's RMAA measurements posted are over spec'd. So, performance wise 30-300 ohm may still be perfectly acceptable, especially off the wallwart? Effectively Ti made a large mistake sticking to RMAA as his specifications?

Quick question, what are the best case numbers for a 15 and 30 ohm load theoretically?

Also, here's an opinion question for you. If someone had purchased a pre-built Mini^3 for $90 shipped years before the E7 was available planning on using them with nothing below 33 ohms, would you have considered the Mini^3 acceptable for the time at that price and with that usage? Furthermore, in a portable rig as mine (portable "desktop" rather than pocket portable?).

The only thing I really am defending Ti on is his intentions and that he probably picked up a strong battery he had laying around. Should he fix estimated based on the accupower (IIRC) most people use? Absolutely. I also think the 3ch may have purpose in the portable segment . . . definitely not the fullsize one though I'd like to see whether it created audible problems.

BTW, I saw Krueger's post on the thread you made in Hydrogenaudio fairly interesting. :)

The issue isn't Ti Kan being mislead by RMAA. It's that someone representing himself as an expert in audio design, offering his designs to thousands of potential DIY builders, and selling components, should know what specs are realistic and what's a reasonable result from RMAA.

In my opinion, Ti either doesn't understand the engineering required to design and measure a decent headphone amp, or if he understands it, he's chosen to intentionally mislead people who spend their time and money on designs that don't work as he advertises. Otherwise, if they're all just honest mistakes, why won't he correct the erroneous information? To me, it seems he's trying to protect his mythological, but fundamentally flawed, 3 channel designs.

By isolating the input and output grounds Ti has made the Mini3 perform even worse for the reasons I've mentioned. And if the Mini3 used a real star ground, the crosstalk would be better still, as you would eliminate the output impedance of the OPA690 and its associated ferrite bead.

Using a virtual (3rd channel) ground can be useful in portable gear. But Ti Kan is promoting it as better when it's clearly not. His claims of lower output impedance, less distortion, and less crosstalk are false and misleading. The Cmoy Amp proves this.

I've done more testing of crosstalk on various amps and my best case estimates in the previous post need some revision. The Benchmark DAC1 achieves 92 dB crosstalk into 150 ohms which is only about 4 mOhms and almost all of that is the jack and plug. But it has the advantage of 1/4" gold output jacks and I was using a high-end Neutrik plug cleaned with Deoxit Gold. With 1/8" jacks, the best I can get is around 88 dB which is around 6 mOhms, and again, almost all of that is from a carefully optimized jack and plug.

The amps above have the advantage their feedback loop is referenced to the star ground which has an extremely low impedance path to the output jack. So any other ground impedance is almost entirely canceled out. The Mini3 doesn't have this advantage because, as I've explained, the inputs are not referenced to the output ground.

But the above does prove a high quality jack and plug can be down in the range of 6 mOhms. So re-doing my "best case" estimate from earlier, you get:

So why did I measure 150 mOhms instead of closer to 56 mOhms? I suspect the OPA690 is not performing anywhere close to its theoretical ideal in Ti's implementation, the output jack probably was closer to its 50 mOhm spec than the ideal conditions with new parts I measured above, and the copper and/or ferrite bead could have easily been higher as well.

As for the pre FiiO days, I don't think you can get all the parts for a Mini3 for $90. But even pre-built at that price I wouldn't buy one. I'd rather have a $39 4556 Cmoy with 2 batteries.

And you bring up a good point about the "full size" AMB designs. How does the Beta22 perform in 3 channel form? I suspect significantly worse than the 2 channel version. And, given Ti's rather optimistic specs for the Mini3, I do wonder about the published specs for all his other designs.

It's interesting to me, despite all the objective evidence he's wrong, many still defend Ti's numbers. There's apparently a strong need to "believe" as I explain in the Subjective vs Objective article. If you have further questions about your personal gear, feel free to contact me privately using the link in the right column.

I have built several AMB products, including the Mini3, and they all sound fine to me. Ti Kan said you are poisoning DIY audio and I have to agree. Why go after AMB and why test the Mini3 into loads and levels it was never designed for? Oh yeah, to make your own amp look better! As Ti said its now clear what your real agenda is. You probably were planning the O2 before you even reviewed the Mini3. Do you realize you are hurting a man's income with your hidden agenda?

- Verifying objective product and design claims made by companies and people like Ti Kan who are selling things.

- Writing educational articles explaining how headphone gear works, helping people choose the right gear, and showing some of the math and engineering behind audio gear.

- Reviewing lots of products--many of them quite favorably.

How is doing all the above "poisoning DIY audio"? I stand behind what I've published as being reasonably accurate, verifiable, etc. And when someone finds an error, I promptly fix it. That's more than I can say for years of misleading and erroneous information being published by Ti Kan at AMB. If hundreds or thousands of people decided to invest in AMB designs based on highly misleading information what do you call that?

As for already planning the O2 back in May, if that were true why would I give the FiiO E9, FiiO E7, QRV09, and a Cmoy from eBay, largely positive reviews? All of those reviews came after the Mini3. The whole reason I was reviewing all those amps was to hopefully find something I could hold up as being a great value that would meet the needs of most people. But the more I learned, the more I realized there was a void in the market.

I don't make any money off the O2 and it's not exactly resume material as I did it under my blog alias. It was created to fill a void, and considering close to 1000 O2 boards (including a group buy for over 150 in Hong Kong) have been ordered, it seems I may have hit the mark. I don't get a penny from any of it.

As for your AMB products "sounding fine" that's great. If they make you happy enjoy them. Just know our ears often deceive us in unavoidable ways. So just because you think they sound good doesn't mean my measurements are wrong. I talk about that in my Subjective vs Objective article.

As for testing the Mini3 with "loads it was never designed for" that's been addressed many times. Please see the previous comments above and the first part of the Tech Section of the review. The majority of portable headphones are 16 ohms, and the majority of portable headphone amps are rated into 16 ohms. Many BA IEMs dip below 16 ohms. If I don't test at 16 ohms, how can I verify the claims from all those companies that rate their gear at 16 ohms? To give the Mini3 the benefit of a doubt I also tested it at 33 ohms--just like Ti Kan claims he did--and it didn't come close to meeting his claimed specs. I also ran most of my tests at only 400 mV--a level any iPod can manage into 16 ohms. So how is that unreasonable?

As for hurting Ti Kan's income I'm sorry if that's the case but is one person's income more important than hundreds or thousands of people buying AMB products based on erroneous claims? Plus, Ti Kan could have made a significant profit by selling many of the 1000 or so boards already ordered. He's welcome to profit from the O2 design. But, instead, he chose to come out against the O2 and discuss it solely on the only two forums where I couldn't respond.

"I have built several AMB products, including the Mini3, and they all sound fine to me. "

Sounding fine is subjective. Compared to few other portable amps (Pimeta, Corda Move...)I would rate the Mini3 at the bottom of the heap... I never understood why anyone would pay so much money for that amp, let alone spend time building one.

I've built the Mini3 and the CMOY BB. I liked the CMOY, and had high hopes for the Mini3. To be honest, it turned out to be rather dissappointing, after spending a lot more money. The case and engraved panels are nice, though.

The whole thing ends up being more trouble than it's worth, and it's sat in my closet unused.

I recently came across your blog and it is simply amazing! None of the mambo jambo that is so prevalent among head-phile forums, just plain scientific proof.

I used to admire Ti Kan because he seemed like a honest, no nonsense kind of guy. However, I was never satisfied with the sound I got out of the Mini3 I built. In audiophile terms (which I loathe), the sound is very coloured, in a bad way. All my headphones/IEMs are low impedance, ranging from 16ohms to 32ohms and there was ALWAYS sibilance in ANY sort of music, I eventually blamed it on my setup (DAC/source, cables(lol) or the headphones itself). I ended up building a CK2III amp then a Gamma1+Gamma2 after which I gave up on audio all together because it was just too much effort - it became more like a critical listening test. And now, I can safely assume all the sibilance on the mini3 came from the distortion of the amp.

On the side note, I really admire what you are doing despite being banned from both Head-fi and Ti Kan's forum. Nowadays, if one is not with the majority or the general consensus, one is normally considered wrong. I think you can consider starting a Facebook page about the matter or even yourself to further your cause and also guide some newbies. I have came across a Singapore forum where mentioning of prices is not allowed because the forum was started by a shop (jaben.net) and everyone accepts it. But hey, we don't even have freedom of expression to begin with. Lol. It's just sickening.

@Kayden, Thanks for the comments. It took a while, but lots of people seem to be losing interest in AMB designs. I've had lots of private messages regarding the issue and it's become apparent Ti Kan has been intentionally publishing misleading measurements for years for most or even all of his designs. That's not fair to all those people who spent a lot of time and money based on false information.

Check back for the desktop version of the O2 amp which will hopefully have a high quality reasonably priced integrated DAC option that, unlike the Gamma DACs, will do 24/96 over USB and not have design problems like nasty electrolytic coupling caps in the signal path.

Very nice article. I definetly love all the information providing here. About those Anonymous, I can't stop thinking they are just Ti Kan himself or his minor trying to flame you. I did trying to measure the high performance mini^3 (I got it from MisterX) in my friend audio lab. The measure result is very similar to what you have here.(~2% difference)

Hi interested to know how "A Cmoy can be worse than no amp at all in many applications"?

I'm currently using a cmoybb by JDS Labs with higher current option (a second TLE2426 rail splitter). It was designed with headphone impedance between 18 and 50 ohms in mind. Currently using triplefi (32 ohms) and AH-D5000 (25 ohms) just purchased & in the mail. Should I have any problems running these phones?

If an amp isn't needed for more power or a lower output impedance, adding one can't help and is more likely to make things worse. All amps add some level of noise and distortion. Some have high output impedance, limited current capability, and other problems.

You don't say what your source is so I can't comment if you would be better off without the Cmoy. Both of your headphones are fairly easy to drive, which is a good thing, as the JDS Cmoy has very limited current capability. It also suffers from being a virtual ground design (see my Virtual Ground article).

For example if your source is the Sansa Clip+ it can already drive both of your headphones with power to spare. And it already has a low output impedance. No amp is needed and adding a Cmoy to the Clip+ with those headphones is a step backwards. If you had 300 ohm Sennheiser HD600s, then you might want an amp with a higher output voltage if the Clip+ by itself wasn't loud enough.

thank you for reply. I use a phone a galaxy s infuse with voodoo sound. Not sure what the output impedance is, but compared to the cmoybb it definitely rolls of the highs with the triplefi & maybe less bass, so I assume that makes the output impedance more on the phone. Also just use notebook as a source again not sure of output impedance. Would the notebook be ok if output impedance is low? I don't notice any noise or distortion from the cmoybb. I also have an external usb soundcard 'X-Fi' might be better?

Some phones, including some Samsungs, have relatively poor headphone outputs so there could be some benefit to the Cmoy. Creative makes several different X-Fi USB DACs. I've only tested the X-Fi Go Pro (see my review). I've heard mixed things about all of them.

To answer your other (unpublished) questions, I'm not familiar with all of the JDS designs, but the ones I've seen should have a low enough output impedance. Because both of your headphones need very little power, and you already have a low impedance source, you don't need an O2.

If you get tired of carrying around a Cmoy to use with your phone, consider the FiiO E5/E6 or a Clip Zip--all of which are much smaller and the Clip is self contained saving the battery life on your phone and making life simpler. You'll get as good of, or possibly even better, sound from a Clip than your phone and Cmoy combo.

The Mini^3 can't drive load's under 30 Ohms, ok, so don't use it for load's under 30 Ohms. Yes, it should have been specified on the mini^3 page.

For loads between 30-100 Ohms the mini^3 is a decent amp, and it can drive the AKG K500 (100 Ohms) very loud. It doesn't sound as good as most 2 battery cmoy's (or at least one one's i've build), and I'm glad someone pointed this out as it's often touted as an "upgraded" cmoy. In reality it's a slight downgrade.

However you were a very harsh on what is a decent amp if size and ease of use (the incorporated charger) is your main concern. Furthermore, what idiot would spend 180 for a mini^3, mine cost's about 60 plus batteries.

I think your a bit angry for being banned from ABM and Head-fi. Please try to be more objective in the future.

For what it's worth I wrote this review before being banned from either AMB or Head-Fi. So that argument isn't valid. And, to be blunt, measurements don't lie. The Mini-3 has some serious design flaws that can effect how it sounds.

You say the Mini-3 can be built for $60, but just the front and back panels, case and bare PC board with no parts cost more than $60. When I wrote this article and followed the links on the AMB website to buy an assembled Mini-3 it was $180.

You say "what idiot would spend $180 for a Mini-3". That I have to somewhat agree with. The FiiO E11 is cheaper, measures significantly better, is smaller, has more features, and better battery life. And the O2 Amplifier flat blows away the Mini-3 and will drive many more headphones that are well beyond the Mini-3's capability. Even at the mythical price of $60 there are better choices.

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